This invention relates to CVD diamond of high colour suitable for optical applications including gemstones of high colour grade. In particular, the present invention relates to a method of adding a gaseous source comprising a second impurity atom type to counter the detrimental effect on colour caused by the presence in the CVD synthesis atmosphere of a first impurity atom type.
The described method applies to the production of both single crystal diamond and polycrystalline diamond, particularly to single crystal diamond.
All documents referred to herein are hereby incorporated by reference.
Methods of depositing material such as diamond on a substrate by CVD are now well established and have been described extensively in patent and other literature. Where diamond is being deposited on a substrate, the method generally involves providing a gas mixture which, on dissociation, can provide hydrogen or a halogen (e.g. F, Cl) in atomic form and C or carbon-containing radicals and other reactive species, e.g. CHx, CFx wherein x can be 1 to 4. In addition, oxygen containing sources may be present, as may sources for nitrogen and for boron. Nitrogen can be introduced in the synthesis plasma in many forms, such as N2, NH3, air and N2H4, for example. In many processes inert gases such as helium, neon or argon are also present. Thus, a typical source gas mixture will contain hydrocarbons CxHy, wherein x and y can each be 1 to 10, or halocarbons CxHyHalz, wherein x and z can each be 1 to 10 and y can be 0 to 10, and optionally one or more of the following: COX, wherein x can be 0.5 to 2, O2, H2, N2, NH3, B2H6 and an inert gas. Each gas may be present in its natural isotopic ratio, or the relative isotopic ratios may be artificially controlled. For example, hydrogen may be present as deuterium or tritium and carbon may be present as 12C or 13C.
Dissociation of the source gas mixture is brought about by an energy source such as microwaves, RF (radio frequency) energy, a flame, a hot filament or jet based technique and the reactive gas species so produced are allowed to deposit onto a substrate and form diamond.
Single crystal CVD diamond has a range of applications including electronic devices and highly engineered optical devices. The properties of the diamond can be tailored specifically for each application, and in so doing limitations are placed on the details of the synthesis process and the cost of producing the material. International application WO 01/96634 describes the synthesis of high purity diamond suitable for electronic applications, which because of the low levels of impurity in the gas phase of the deposition process and subsequently in the solid also show low absorption and are suitable for the production of “high colour” diamond (that is, material with absorption close to the theoretical limit for impurity free diamond, and thus typically providing colours equivalent to the natural diamond colour grades of D to better than K, where these are colour grades on the Gemological Institute of America (GIA) colour scale, see ‘Diamond Grading ABC’, V. Pagel-Theisen, 9th Edition, 2001, page 61). However, there are economic penalties in providing the degree of control necessary to achieve the low levels of nitrogen used in the method of that invention.
The colour scale of the Gemological Institute of America (GIA), which is the most widely used and understood diamond colour scale, is shown in Table 1. Table 1 is derived from ‘Diamond Grading ABC, The Manual’, Verena Pagel-Theisen, 9th Edition 2001, published by Rubin and Son n.v. Antwerp, Belgium, page 61. The colours are determined by comparison with standards. The determination of the colour of diamonds is a subjective process and can only reliably be undertaken by persons skilled in the art.
TABLE 1Colour on GIAImpression ofScaleColourDColourlessEF*GAlmostHcolourlessIJKLPale yellowishMNVery lightOyellowishPLight yellowQRYellowishSTUVWXYZZ+Fancy Colours*colourless for round brilliants less than 0.47 cts.
The clarity scale of the Gemological Institute of America (GIA), which is the most widely used clarity scale, is shown in Table 2. Table 2 is derived from ‘Diamond Grading ABC, The Manual’, Verena Pagel-Theisen, 9th Edition 2001, published by Rubin and Son n.v. Antwerp, Belgium, page 61. It takes into account both internal and external flaws on a cut diamond. Typically, examination is made with the aid of a 10× magnifier or loupe by an experienced grader with appropriate illumination for the type of defect that is being sought.
TABLE 2Desig-DescriptionnationNotesFlawlessFLFlawless: No internal or external features, withthe exception of extra facets that are not visiblefrom the upper facet; naturals at the girdlewhich neither widen it nor make it irregular;non-reflecting internal growth lines which areneither coloured nor white and do not affecttransparency.InternallyIFLoupe Clean (internally flawless): no inclusionsflawlessand only minor external features, with theexception of small external growth lines.Very veryVVSVery, very small inclusions 1 & 2: very smallslightly1 & 2inclusions which are difficult to see; in the caseincludedof VVS1, these are very difficult to see andthen only from the lower facet or they are sosmall and sufficiently near the surface to beeasily cut away (potentially flawless). In thecase of VVS2, the inclusions are still verydifficult to see. Typical inclusions includeoccasional spots, diffuse, very fine clouds,slight beading on the girdle, internal growthlines and very small fissures, nicks or blowindentations.VeryVSVery small inclusions 1 & 2: smaller inclusionsslightly1 & 2ranging from those which are difficult to see toincludedthose which are somewhat easier to see.Typical inclusions are small included crystalsand small fissures, more distinct small cloudsand groups of dot-like inclusions.SlightlySISmall inclusions 1 & 2: inclusions which areincluded1 & 2easy (SI1) or very easy (SI2) to see; theinclusions are often in a central position, can berecognised immediately and in some cases arealso visible to the naked eye.ImperfectIInclusions 1, Inclusions 2 and Inclusions 3:1 to 3distinct inclusions which in most cases areeasily visible to the naked eye through thecrown; in the case of inclusions 3, stonedurability can be endangered. Typicalinclusions are large included crystals andcracks.
By “high clarity” is meant herein a clarity of at least SI 1 as defined in Table 2, preferably at least VS 2.
The GIA diamond gem grading system is the most widely used grading scale for diamond gems and generally considered the definitive grading scale. For the purposes of this application all gem colour grades are based on the GIA colour grades, and other gem properties such as clarity are likewise based on the GIA grading system. For a given quality of diamond, i.e. material with given absorption characteristics, the colour of a gem also varies with the size and cut of gem produced, moving to poorer colours (to colours towards Z in the alphabet) as the stone gets larger. To enable the colour system to be applied as a material property it is thus necessary to further fix the size and type of cut of the gemstone. All GIA colour grades given in this specification are for a standardised 0.5 ct round brilliant cut unless otherwise stated.
In contrast to growing high purity layers with high colour, synthesis of coloured gemstones, in which deliberate controlled levels of impurities are added to the process, is reported in WO 03/052177 and WO 03/052174. These techniques provide a method for producing CVD diamond layers and CVD diamond gemstones of a range of colours, typically in the blue or brown part of the spectrum.
Nitrogen is a significant impurity in CVD diamond processes. The extent to which it plays a key role in determining the colour and quality of the material is emphasised in the earlier mentioned prior art. Nitrogen is very prevalent, forming the majority of the atmosphere, and commonly being the major contaminant of gas supplies, even those specified as ‘high purity’. It is expensive to remove nitrogen from high purity gas supplies to the levels necessary for synthesis of high colour diamond using the method described in WO 01/96634, which impacts on the cost of the final material, and it is desirable to identify alternative synthesis methods more tolerant of impurities, which are suitable for the production of the thick layers of high colour necessary for the production of gemstones and other selected optical devices.
Diamond containing nitrogen in the form of single substitution nitrogen, present in sufficient concentration to give observable spectroscopic features, is called Ib diamond. The spectroscopic features include an absorption coefficient maximum at 270 nm and, to longer wavelengths, a gradual decrease in absorption coefficient between approximately 300 nm and 500 nm, with signs of a broad absorption band at approximately 365 nm. These features can be seen in absorption spectra of a type Ib high pressure high temperature diamond such as spectrum A in FIG. 1.
Although the effect of single substitutional nitrogen on the absorption spectrum is greatest in the ultra-violet, it is the weaker absorption that extends into the visible region of the spectrum that affects the colour of the type Ib diamond and gives it a characteristic yellow/brown colour.
The UV/visible absorption spectrum of homoepitaxial CVD diamond grown in the presence of nitrogen typically contains a contribution from single substitutional nitrogen with the spectral characteristics described above. In addition to single substitutional nitrogen, homoepitaxial CVD diamond grown in the presence of nitrogen typically contains some nitrogen in the form of nitrogen vacancy centres. When the N-V centre is electrically neutral [N-V]0 it gives rise to absorption with a zero phonon line at 575 nm. When the N-V centre is negatively charged [N-V]− it gives rise to absorption with a zero-phonon line at 637 nm and an associated system of phonon bands with an absorption maximum at approximately 570 nm. At room temperature the absorption bands of these two charge states of the N-V centre merge into a broad band from about 500 nm-640 nm. This absorption band is in the yellow part of the visible spectrum, and when it is strong the crystals can exhibit a complementary pink/purple colour.
The UV/visible absorption spectra of low quality homoepitaxial CVD diamond grown in the presence of nitrogen, may also show a gradual rise in measured absorption from the red to the blue region of the spectrum and into the ultra-violet. There may also be contributions from scattering. The spectra generally contain no other features, apart from those related to single substitutional nitrogen. This absorption spectrum gives an undesirable brown colour and such diamond often contains clearly visible graphitic inclusions.
The absorption spectrum of higher quality homoepitaxial CVD diamond grown in the presence of nitrogen contains additional contributions that are not present in natural, HPHT synthetic diamond or low quality CVD diamond. These include two broad bands centered at approximately 350 nm and 510 nm.
The band at approximately 350 nm is distinct from the broad feature in that region of the spectrum of ordinary type Ib spectrum and distorts the spectrum of ordinary type Ib diamond to an extent dependent on the concentration of the centre responsible relative to the single substitutional nitrogen.
Similarly the band centered at approximately 510 nm can overlap absorption relating to negative nitrogen-vacancy centres and the visible absorption relating to single substitutional nitrogen.
The overlapping of the various contributions to the absorption spectra can cause the bands at approximately 350 and 510 nm to give rise to broad shoulders in the absorption spectrum rather than distinct maxima. These contributions to absorption do however have a very significant effect on the relative absorption coefficients of the diamond at wavelengths in the spectral region between 400 and 600 nm where the eye is very sensitive to small differences. They therefore make an important contribution to the perceived colour of the diamond.
The width and position in the spectrum of these bands can vary. The position of peak maxima is most easily ascertained by using the second differential of the spectrum. It has been found that absorption spectra for homoepitaxial CVD diamond grown in the presence of nitrogen, and in the absence of any second impurity used according to the current invention, can generally be deconstructed into the following approximate components.    1) Single substitutional nitrogen component with an absorption coefficient at 270 nm that is generally within the range 0.4 m−1 and 10 m−1 and an absorption coefficient at 425 nm that generally lies between 0.04 m−1 and 1 m−1.    2) An absorption band centered at 3.54 eve (350 nm)+/−0.2 eve with a FWHM of approximately 1 eve and a maximum contribution to the absorption spectrum generally between 1 and 8 m−1 at its centre.    3) An absorption band centered at 2.43 eve (510 nm)+/−0.4 eve with a FWHM of approximately 1 eve and a maximum contribution to the absorption spectrum generally between 0.2 and 4 m−1 at its centre.    4) A small residual wavelength dependent component of the measured absorption coefficient (in m−1) that is found to have a wavelength dependence of the following approximate form: c x (wavelength in microns)−3 where c<0.2 such that the contribution of this component at 510 nm is generally less than 1.5 m−1.
FIG. 1 shows the absorption spectrum of a brown CVD diamond layer (curve B) and the components into which it can be decomposed. The first step in such a spectral decomposition is the subtraction of the spectrum of a type Ib HPHT synthetic diamond (curve A), scaled so that the residual shows no 270 nm feature. The residual spectrum can then be decomposed into a c×λ−3 component (curve C) and two overlapping bands of the kind described above (curve D).
It has been found that the form of UV/visible spectra of CVD diamond grown using a range of different processes can be well specified by sums of the components described above, with different weighting factors for the components in different cases. For the purposes of specifying the shape of the spectrum the contributions of the different components are given in the following ways.
270 nm: The peak 270 nm absorption coefficient of the type Ib component is measured from a sloping baseline connecting the type Ib spectrum either side of the 270 nm feature that extends over the approximate range 235 nm-325 nm.
350 nm band: The peak absorption coefficient contribution of this band.
510 nm band: The peak absorption coefficient contribution of this band.
Ramp: The contribution of the c×λ−3 component to the absorption coefficient at 510 nm.